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This study investigates changes in extreme rainfall and their return periods during the baseline period, and to identify trends in future extreme rainfall events in Iran. To this end, a combination of data and statistical methods, remote sensing, synoptic analysis, and climate modeling has been used. In this research, to understand Iran's extreme rainfall, data from 232 synoptic stations from their establishment until the end of 2023, as well as daily rainfall data from the CMIP6 model set for the period 2020-2040, have been used. Additionally, the GrADS software and data on geopotential height, u and v components, sea level pressure, and relative humidity of the atmosphere at 100 to 250 hPa levels were used for synoptic analysis of widespread rainfall. The statistics showed the 155 mm 24-hour total rainfall occurred in Izeh station in March 2019 ranked first during the study period while the 142 mm 24-hour rainfall of Poldokhtar station ranked the second. The estimated return period of the heavy rainfall of late March and early April 2019 in western Iran varies between 7 to 200 years across the studied stations. The estimated chance of recurring 155 mm 24-hour heavy rainfall at Izeh station is 96 years but it is 56 years for the 142 mm rainfall in Poldokhtar station. In the case of January 2020 heavy rainfall occurred in southern and southeastern Iran, the estimated return period of the 176 mm 24-hour total rainfall measured in Qeshm and Minab stations of Hormozgan province was 115 years whereas it was 10 years for the 107 mm of rainfall at Rask station in Sistan and Baluchestan province. The calculation of the return period for extreme rainfall during the study period was performed using the General Extreme Value technique.Statistical results showed that the maximum 24-hour rainfall was recorded at Bandar Anzali station with 353 millimeters in the year 1962. Geographically, in addition to the northern coastal areas, especially stations located on the western shores of the Caspian Sea, regions such as Sardasht in the northwest, Ilam, Yasuj, and Kouhrang in the west and southwest, as well as Bandar Abbas and Minab stations in the south of Iran, have recorded heavy 24-hour rainfall. In contrast, stations located in central regions, especially Yazd Province (such as Mehriz) and desert edge stations in Kerman Province (such as Shahdad), have received the minimum 24-hour maximum rainfall. The calculation results of the return period for maximum 24-hour rainfall showed that the maximum 24-hour rainfall in the northern coastal region is 353 millimeters with a return period of 200 years and more. The eastern and northeastern regions have a maximum of 65 millimeters with a 50-year return period, the central region has a maximum of 108 millimeters with a 50-year return period, the northwestern region has a maximum of 185 millimeters with a 100-year return period, the western and southwestern regions have a maximum of 282 millimeters with a 100-year return period, and the southern and southeastern regions have a maximum of 211 millimeters with a 100-year return period. In many regions of Iran, extreme rainfall of 30 to 70 millimeters can occur within short return periods of 2 to 10 years.
The synoptic analysis of the consecutive downpour of late March and early April 2019 occurred in the west and southwest of Iran and the successive heavy rainfall of January 2020 that occurred in the southern and southeastern Iran and heavy rainfall of July 2022 that occurred in the tehran(Imamzadehdavood)was related to the blocking synoptic systems and the associated atmospheric turbulences established in these areas. In the heavy rainfall of March 2019, the formation of a strong cut-off low centered over Iraq sank the higher latitude cold air into the system that in turn intensified it that favors absorbing intensive moisture from the Mediterranean Sea. The penetration of the system,s flanks into the Red Sea also aid the system to absorb hot and humid air from the low latitudes that fed consecutive days of heavy rainfall in southwestern Iran that lays in the forehead of the cut-off low. In the case of heavy rainfall of January 2020 that occurred in the south and southeast of Iran, a very deep trough was formed over the area due to the formation of an Omega-shape blocking system. The advection of cold air from the higher latitudes and the hot and moist air from the southern water bodies of Iran into the system causes intensive heavy rainfall in the area which is situated in front of the trough. From a synoptic point of view, in early August 2022, due to the departure of the atmosphere of most regions of Iran from the influence of high pressure and the northward displacement of its tabs, the formation of the blocking synoptic systems and being situated under the positive vorticity resulting from its a strong trough over the north of Iran and the Emamzadeh Davood region, strengthening Thermal Low-Pressure and the arrival of warm and humid currents from the south-southern water areas as a result of the activity of the monsoon system and finally the rise of hot and humid air as a result of strong convection activities and also the rise from the heights, heavy rains and floods in different regions of the country and especially in the Emamzadeh Davood region. Tehran has been formed. The intensity and amount of rainfall received in different regions have been influenced by the amount of convective activities as well as environmental characteristics such as the presence of different heights.
The results of examining future extreme rainfall events in Iran during the period 2020-2040, based on various SSP126, SSP245, and SSP585 scenarios, showed that overall, there is no significant and directional positive or negative trend for rainfall. Different positive and negative trends with varying intensities are expected in different regions of the country for these indices. The results of the future total rainfall trend in Iran under the SSP126 scenarios showed that for 82% of the regions in iran , the PRCPTOT index trend is negative and non-significant, and for 9%, it is negative and significant. The status of the future total rainfall index under the SSP245 scenario showed that the rainfall trend at no station in Iran will be positive and increasing. For 99% of the regions, the Mann-Kendall statistic values are in the negative range but non-significant, and for the remaining 1%, a negative and significant trend is predicted. The results of extreme indices trends under the SSP585 scenario showed that, in this scenario, the number of regions with a positive and significant trend in Iran is higher compared to the SSP2.6 and SSP4.5 scenarios. In contrast to the other two scenarios, in the SSP585 scenario, the trend slope of the indices is more positive, and the frequency of regions with a positive trend (both significant and non-significant) is higher than those with a negative trend (both significant and non-significant). The Precipitation total index (PRCPTOT) is positive for 82% of the regions in Iran, consistent with the extreme rainfall indices.
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